1. Field of the Invention
The present invention relates generally to a vehicular braking system actuated by pneumatic pressure, and more particularly, to an apparatus for equalizing pressure and absorbing shock in such a system and that employs a sealing covering to prevent water droplets and particulate contaminants which are present in the system from adversely affecting its operation.
2. Description of the Prior Art
Heretofore, apparatus for equalizing pressure and absorbing shock has not been available in pneumatic braking systems of the type commonly employed in trailer trucks and the like. In such systems the operation of one of a pair of brake actuators may be interrupted by a resistance factor which will either stop or temporarily delay its function while the other actuator proceeds to operate normally. Variations in the resistance factor can be caused by road surface conditions, variations in the air pressure in the tires, or unequal tire treads. This can cause erratic or uneven operation, or in some cases a complete malfunction of the brakes.
In a pneumatic braking system having drum-type brakes, each drum is actually an imperfect "out-of-round" circle with a so-called "high spot." Consequently, when the brakes of the truck are applied, the brake shoes pass over all high spots until the drum ultimately stops with one shoe at a high spot. Since the size and location of the high spot differs for each pair of brake assemblies, one wheel generally tends to "lock-up" before the other wheel. Should this occur the truck typically swerves or skids to one side. The problem becomes more severe in emergency situations when the brakes are applied with an increased force.
In the past it has been observed that prior to the occurrence of this lock-up condition, a shock wave of increased pressure reverberates along the air lines through which the air flows and that by absorbing or equalizing the shock wave between any pair of wheels, lock-up will not occur.
Attempts to solve this "lock-up" problem in automobiles having hydraulic brake systems are found in the U.S. Pat. No. 3,430,660, entitled "Pressure Equalizer Apparatus for Hydraulic Brake Fluid Systems" by R. E. Mitton; U.S. Pat. No. 3,757,825 entitled "Pressure Equalizing Device for Fluid Pressure Systems" by R. H. Givens and T. P. Spero, the last named being the inventor of the present invention and copending patent application Ser. No. 419,643, filed Nov. 28, 1973, entitled "Pressure Equalizing and Stabilization Device for Hydraulic Brake Systems," also invented by T. P. Spero. The device described by Mitton employs a hollow resilient bulb having a central recess and that is retained in a cavity with bulging sidewalls. Access to the recess is through a relatively small orifice. As a consequence it was found that air tends to become trapped within the recess, causing the device to react sluggishly. Another disadvantage of the Mitton device is that the hydraulic pressure is not applied uniformly to the shock-absorbing bulb so that the device does not respond in a repetitive and effective manner to variations in the hydraulic pressure. Neither that device nor the devices described in U.S. Pat. No. 3,757,825 or Ser. No. 419,643 employ a protective covering over its shock-absorbing member in a manner to seal, and hence isolate, it from external contaminants.
In this regard it has been found that devices of the type described which are used in hydraulic braking systems are not capable of operation in pneumatic braking systems. This is probably due to the presence of water droplets and particulate contaminants in the lines that couple the pneumatic power source to the air brake actuators through a release valve. More particularly, there is a decrease in temperature associated with the release and consequent expansion of air as it flows through the pneumatic braking system. Hence, even in an ambient environment that is slightly above the freezing temperature, the water droplets which form during condensation and remain in the air lines freeze as the air brakes are applied. This renders the safety braker device of the prior art inoperable in cold climates. In addition, the steel connecting air lines of pneumatic braking systems are typically filled with flakes of rust particles formed during the rusting process. Such particles have a tendency to wear the internal parts of a device inserted in the air lines, and hence make it susceptible to failure after a relatively short lifetime.
In 1975 the federal government forced the trucking industry to employ electronic computer controlled safety braker devices in the hopes of solving the wheel lock-up problem. However, such computer controlled devices are now suspected to be at fault in literally scores of accidents. Because of this a blue-ribbon advisory panel has requested that the government suspend the computer controlled anti-lock requirement.